Nervous system 2
Monacia Williams, Contributor

Last week, we began our study of the nervous system. I hope you have revised this lesson and have practised the drawing of the diagrams. By now you should be able to provide definitions for all the new words that came in the lesson. Remember that whatever you are learning now, you should try to learn it well because you may not have a chance to learn it again before you face the exams.

This week as we continue, let us look at the functions of the nervous system.

These are:

• It controls our actions, both voluntary and involuntary.
• It coordinates different parts of the body so that they work together in order to achieve the necessary responses. In order to do this:
• It receives information from our sense organs.
• It makes us aware of changes in our environment.
• It allows us think and to make decisions based on our experiences.
• It allows us to react quickly to happenings that might be dangerous. These reactions do not involve thought and are known as reflex actions.

What is a reflex action?

A reflex action is an automatic, unchanging response to a stimulus. This response is carried out by a reflex arc. The reflex arc comprises three different types of neurones. Do you remember what these are? They are the sensory, the intermediate and the motor. If you are not yet familiar with them, go back to last week's lesson and revise. The following diagram shows the arrangement of these neurones to form the reflex arc:

This is another diagram that must be learnt, both to draw and to label. Learn the different parts given in the list below the diagram and it will make the drawing and the labelling much easier.

Diagram of the reflex arc

Parts of the reflex arc

There are five parts to the reflex arc. These are:

• The receptor - receives the stimulus
• The sensory neurone - carries the stimulus from the receptor to the intermediate neurone in the spinal cord or the brain
• The intermediate neurone - carries the impulse from the sensory neurone in the upper region of the spinal cord or brain to the motor neurone in the lower region
• The motor neurone - carries the impulse from the spinal cord or brain to the muscle.
• The effector - responds when the impulse reaches it
• Note that there are three synapses in the reflex arc: one between the sensory and the intermediate; one between the intermediate and the motor; and the last one between the intermediate and the muscle.

Types of reflex actions

There are two types of reflex actions. These are:

• The spinal reflex - where the intermediate neurone is in the spinal cord, for example the knee-jerk reflex, removing your hand from a hot surface, sitting on an open pin.
• The cranial reflex - where the intermediate neurone is in the brain, the pupil reflex.

Your syllabus requires that you learn the operations of the knee-jerk reflex and the pupil reflex. Use the information above to help you with this. Note that in the pupil reflex, the stimulus is light, which is shone directly into the eye of the individual. In dim light, the radial muscles of the iris contract and the circular muscles relax, contracting the iris and making the pupil wider. This allows more light to enter the eye providing a stronger stimulus for the retina. In bright light, when the light is shone directly into the eye, the radial muscles relax and the circulars contract, making the iris larger and the pupil smaller. This reduces the amount of light that goes into the eye, preventing the retina from becoming damaged due to over stimulation. Remember that this is an example of a cranial reflex.

In the knee jerk, the stimulus is a tap just below the kneecap, causing the thigh muscle to stretch, stimulating the sensory neurone which then sends the impulse to the intermediate neurone in the spinal cord. A message is taken back to the muscles, causing them to contract, raising the leg. Remember, this is an example of a spinal reflex.

Try out these two experiments on your friends and see if they work! What happens when you eat and talk at the same time? Do you remember? Yes, food can go into your trachea because the epiglottis is open. What happens then? You cough. Do you think about coughing before you cough? No, you don't. Coughing is involuntary or automatic. This is another example of a reflex action.

The importance of reflex actions

• Evasive actions can be taken quickly in order to allow the animal to escape danger.
• Everyday bodily functions can take place, for example breathing, swallowing food and blinking.

See you next week as we continue our study of the nervous system.

Monacia Williams teaches at Glenmuir High School. Send questions and comments to kerry-ann.hepburn@gleanerjm.com